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Short-term memory in Down syndrome: Applying the working memory model

Christopher Jarrold, and Alan Baddeley

This paper is divided into three sections. The first reviews the evidence for a verbal short-term memory deficit in Down
syndrome. Existing research suggests that short-term memory for verbal information tends to be impaired in Down syndrome, in contrast to short-term memory
for visual and spatial material. In addition, problems of hearing or speech do not appear to be a major cause of difficulties on tests of verbal short-term
memory. This suggests that Down syndrome is associated with a specific memory problem, which we link to a potential deficit in the functioning of the
'phonological loop' of Baddeley's (1986) model of working memory. The second section considers the implications of a phonological loop problem.
Because a reasonable amount is known about the normal functioning of the phonological loop, and of its role in language acquisition in typical development,
we can make firm predictions as to the likely nature of the short-term memory problem in Down syndrome, and its consequences for language learning. However,
we note that the existing evidence from studies with individuals with Down syndrome does not fit well with these predictions. This leads to the third section
of the paper, in which we consider key questions to be addressed in future research. We suggest that there are two questions to be answered, which follow
directly from the contradictory results outlined in the previous section. These are 'What is the precise nature of the verbal short-term memory deficit
in Down syndrome?', and 'What are the consequences of this deficit for learning?'. We discuss ways in which these questions might be addressed in
future work.

Jarrold, C, and Baddeley, A. (2001) Short-term memory in Down syndrome: Applying the working memory model. Down Syndrome
Research and Practice, 7(1), 17-23. doi:10.3104/reviews.110

1. Outline

This paper has three aims. First, to review the evidence for a verbal short-term memory deficit in Down syndrome, and in doing so to consider whether any
observed deficit really reflects an underlying memory difficulty, or is instead caused by other factors. Second, to explore the possible implications of a
verbal short-term memory deficit for individuals with Down syndrome, and finally, to highlight key questions that remain to be addressed by future work in
this area. This paper draws heavily on material covered in an earlier paper published in Down Syndrome Research and Practice (Jarrold, Baddeley & Phillips, 1999), and the reader is referred to this
previous account for a fuller treatment of many of the issues raised here.

2. A Review

Short-term memory is the ability to 'hold in mind' information. For example, one might try to remember the string of digits in a telephone number
while moving to the phone and dialling. It differs from long-term memory, where information, although not necessarily forgotten, is not always consciously
available until recalled. Verbal short-term memory refers to the ability to maintain verbal items such as words or digits, and is most often assessed using
'span' paradigms in which individuals are asked to remember increasing numbers of words or digits. The maximum number of items that an individual can
remember, in correct order, is their span. Adults typically have a span of around seven items (Miller, 1956), but
among individuals with Down syndrome verbal short-term memory spans are usually considerably lower than this.

In fact many studies have shown that individuals with Down syndrome have lower verbal short-term memory spans than would be predicted by their level of
verbal ability or their general level of intellectual function. For example, Kay-Raining Bird and Chapman (1994) compared the digit spans of a group of 47 individuals with Down syndrome and a
corresponding number of typically developing children of an equivalent level of intellectual functioning. Among these controls digit spans ranged from 3 to
6, with a span of 5 being most common. Although 1 of the individuals with Down syndrome achieved a digit span of 6, the average level of performance was
considerably lower than that seen among controls; the most common span for individuals with Down syndrome was 4 and a number of individuals had a span of
2.

It is also worth noting that the problems experienced by the majority of individuals with Down syndrome on tests of verbal short-term memory are
relatively specific, in that they do not appear to extend to all aspects of short-term memory. Short-term memory can be assessed for visuo-spatial
information, as well as for verbal material; typically using a 'Corsi span' test (after Corsi, cited in Milner,
1971). In this task individuals watch while the tester taps out a series of different spatial locations on a board, and then have to repeat the sequence
in the correct order themselves. As with digit span, the number of items to be remembered can be gradually increased to determine the maximum level of an
individual's memory ability. The majority of studies which have contrasted digit and Corsi span among individuals with Down syndrome, in some cases
relative to appropriate controls, have shown that individuals with Down syndrome are impaired only on digit span (Azari
et al., 1994; Jarrold & Baddeley, 1997; Jarrold, Baddeley & Hewes, 1999; Wang & Bellugi, 1994; though see Haxby, 1989; Vicari, Carlesimo & Caltagirone, 1995). In other words, the individuals'
visuo-spatial short-term memory is relatively unimpaired, in contrast to their verbal short-term memory performance.

However, this, in itself, does not necessarily prove that individuals with Down syndrome have a specific verbal short-term memory problem. Their
relatively poor performance on digit and word span tasks might arise for reasons which have comparatively little to do with memory. One obvious explanation
for poor verbal short-term memory performance is that hearing difficulties, which are known to be relatively common in Down syndrome (Davies, 1996), affect individuals' ability to properly encode the items being presented. Verbal short-term memory tasks
typically involve auditory presentation of the to-be-remembered items, and clearly if one has difficulty in hearing and identifying these items then
one's memory for them is likely to be severely reduced.

This issue has been addressed in a number of elegant studies carried out by Marcell and colleagues. Marcell and Armstrong (1992) found that visual presentation of digits in a span task did not improve performance among
individuals with Down syndrome, which contrasts with what would be expected if hearing difficulties were affecting the encoding of stimuli. Marcell, Harvey
and Cothran (1988) found that presenting auditory stimuli
through headphones also had no beneficial effect on the short-term memory performance of individuals with Down syndrome. Although this manipulation was
designed to assess individuals' susceptibility to 'distractibility' in verbal short-term memory tasks, one might expect headphones to produce a
higher quality auditory signal than normal speech. Finally, Marcell and Cohen (1992) found
that direct measures of individuals' hearing ability did not predict their performance on a digit span task (see also Jarrold & Baddeley, 1997). Again this is at odds with what one would expect if hearing
difficulties were a major constraint on verbal short-term memory in Down syndrome. Having said this, Marcell and Cohen (1992) did find that the extent of hearing loss was related to the time taken by individuals with
Down syndrome to identify digits, which may have some indirect effect on verbal short-term memory performance (Rabbitt, 1968).

A second possible explanation of verbal short-term memory difficulties in Down syndrome concerns problems of spoken articulation, which again are known to
be associated with the condition (Dodd, 1975; Gibson, 1978). In word and digit span
tasks, individuals typically give their response by repeating the presented list in full. It seems intuitively likely that an individual with difficulty in
planning and/or executing a sequenced spoken response would show reduced verbal short-term memory spans as a result of these problems. Indeed work by Cowan
and his colleagues (e.g., Cowan, Day, Saults, Keller, Johnson &
Flores, 1992) has shown that the time taken by typically developing individuals to produce their response in a verbal short-term memory task has a direct
effect on their memory performance.

Once again Marcell and colleagues have done much of the work aimed at addressing this particular issue. In two studies Marcell and Weeks (1988) adapted digit and word span tasks so as to remove the need for individuals with Down
syndrome to give a full spoken response at recall. In their first study, individuals recreated the to-be-remembered list in a digit span task by ordering a
series of numbered blocks. In their second study, individuals gave their response to a word span task by pointing to pictures of the presented words in the
correct serial order. In neither case did these manipulations improve the performance of individuals with Down syndrome relative to the standard recall
procedure. If removing the need for a spoken response does not remove the difficulty with the task, then clearly this suggests that problems of speech are
not a major factor in determining verbal short-term memory performance. A similar conclusion follows from the results of a recent study from our own research
group. Jarrold et al. (2000) found that individuals with
Down syndrome had lower verbal short-term memory spans than a comparison group of individuals with moderate learning difficulties, despite being able to
repeat pairs of words in an articulation test at a similar rate to these controls. Again this implies that poor verbal short-term memory is not primarily due
to problems of spoken articulation in Down syndrome.

In sum, individuals with Down syndrome tend to perform relatively poorly on tests of verbal-short term memory; a problem which appears to be specific to
verbal tasks and does not extend to all short-term memory tests. Although one might well expect that problems of hearing and speech, which are associated
with Down syndrome, are likely to be the cause of much of this difficulty, there is actually relatively little evidence to support this suggestion. Of
course, this raises the question of what exactly is causing the relatively poor verbal short-term memory performance seen among individuals with Down
syndrome? One possible answer is that there really is a fundamental short-term memory problem associated with Down syndrome. A number of authors have
advanced this view (e.g., Broadley, MacDonald & Buckley, 1995; Comblain, 1996; Das & Mishra 1995; Hulme & Mackenzie, 1992; Jarrold &
Baddeley, 1997; Kay-Raining Bird & Chapman, 1994; Mackenzie & Hulme, 1987; Varnhagen, Das & Varnhagen, 1987; Wang & Bellugi 1994), and have tended to do so with reference to Baddeley and Hitch's
'working memory model'.

The working memory model, outlined initially by Baddeley and Hitch (1974) and
refined by Baddeley (1986), represents an attempt to explain and account for a range of phenomena associated with
short-term memory performance in both typically and atypically functioning individuals. A key feature of the model is that it suggests that separate systems
are dedicated to the short-term storage of verbal and visuo-spatial information. These are termed the 'phonological loop' and the 'visuo-spatial
sketchpad' respectively. Broadly speaking there are three lines of evidence to support this account. First, the type of experimental manipulations which
affect verbal short-term memory performance have little or no effect on visuo-spatial short-term memory performance, and vice versa (e.g., Brandimonte, Hitch & Bishop, 1992). Second, studies that have
examined the pattern of brain activity in individuals performing verbal and visuo-spatial short-term memory tests have suggested that different locations in
the brain are associated with these two sets of tasks (see Baddeley, 1993). Finally, and in line with the
previous point, there are cases of adult neuropsychological patients who have suffered brain damage which has had a selective effect on verbal rather than
visuo-spatial short-term memory, and vice versa. All three lines of evidence point to the separateness of the short-term memory systems responsible for the
storage of verbal as opposed to visuo-spatial information; a suggestion which itself is consistent with the possibility of a selective verbal short-term
memory problem in Down syndrome. In other words, according to the working memory model, Down syndrome may be associated with some form of 'phonological
loop' deficit.

3. Implications

The claim of a phonological loop deficit in Down syndrome amounts to more than a restatement of the data reviewed above, because it has, at least, two
important implications. The first follows from our knowledge of the likely consequences of a phonological loop problem and the second from our understanding
of the normal functioning of this system.

There is considerable evidence to suggest that the phonological loop plays an important role in the development of aspects of language, and in vocabulary
acquisition in particular (see Baddeley, Gathercole & Papagno, 1998 for a
review). The first evidence for the role of the loop in vocabulary acquisition came from work with a neuropsychological patient - PV - known to have impaired
phonological loop functioning. Baddeley, Papagno and Vallar (1988) studied
PV's ability to associate known target words with two types of material: other known words or novel 'nonsense' word stimuli. They showed that
while PV was perfectly able to learn associations between pairs of known words, her ability to pair words with nonsense words was drastically impaired. These
data were seen as being consistent with the view that the phonological loop is involved in maintaining the novel phonological form of a newly experienced
word during vocabulary learning. Subsequent work by Gathercole and her colleagues, among others, has shown that children's verbal short-term memory
functioning relates to their ability to learn the phonological form of new word stimuli in vocabulary learning tasks (e.g., Gathercole, Hitch, Service & Martin, 1997). These
results imply that individuals with Down syndrome should have problems in learning the phonological form of words during vocabulary acquisition, given their
apparent problems in verbal short-term memory.

In addition to knowing something about the probable consequences of a phonological loop impairment, our knowledge of the workings of the loop has
implications for the precise type of problem experienced by individuals with Down syndrome, which in turn has potential implications for intervention
strategies. Evidence from work with typically functioning adults has led Baddeley (1986) to suggest that the loop
can be divided into two components. One is a passive store, which maintains information in a phonological form, but which is subject to forgetting due to the
decay of verbal memory traces. A second process - rehearsal, offsets this decay. By subvocally repeating to-be-remembered information an individual can
prolong the maintenance of material within the phonological loop.

The importance of rehearsal is shown by the effects of blocking this process. If adults are asked to continually repeat a word out loud during a verbal
short-term memory task, a process known as 'articulatory suppression' which blocks rehearsal, then spans are markedly impaired (Baddeley, Thomson & Buchanan, 1975). In addition, an insight into the nature of
rehearsal is shown by the fact that the faster an individual is able to rehearse, as indexed by the speed with which they can articulate words out loud, the
greater their verbal short-term memory span (e.g., Baddeley, et
al. 1975). This is thought to reflect the time-limited nature of the store within the phonological loop - the faster one rehearses the more words
one can maintain in the store without any being subjected to forgetting. This is further reflected in the 'word length effect'; individuals show
higher spans for words with a short spoken duration, which take relatively less time to rehearse, than for words with a longer spoken duration (Baddeley et al., 1975). Clearly, given the importance of rehearsal in
determining phonological loop efficiency, it is possible that the problems experienced by individuals with Down syndrome in verbal short-term memory tasks
are due to a deficit in this process. If this is the case, then one might be able to target interventions towards this problem, in the hope of improving
rehearsal inefficiency. Indeed a number of studies have attempted to improve or train rehearsal in individuals with Down syndrome (Broadley & MacDonald, 1993; Broadley, MacDonald & Buckley, 1994; Comblain,
1994; Laws, MacDonald & Buckley, 1996; Laws, MacDonald, Buckley & Broadley, 1995; for more details on
rehearsal training see Conners, Rosenquist and Taylor (2001).

The claim of a phonological loop deficit in Down syndrome therefore has implications for our understanding of Down syndrome, and for possible intervention
strategies. However, it is important to note that the accounts outlined above, although theoretically plausible, are not fully supported by empirical
evidence. First, it is far from clear that vocabulary abilities in Down syndrome are as poor as one would predict given the evidence of a phonological loop
deficit and the known importance of the loop in vocabulary acquisition. For example, Rondal (1995) reports the case
of Françoise, an individual with Down syndrome with poor verbal short-term memory, but relatively superior vocabulary and other language skills. More
generally, although researchers are not fully agreed about exactly how strong vocabulary abilities are in Down syndrome, it appears that vocabulary tends to
be relatively advanced in comparison to other aspects of language (see Chapman, 1995). This should not be the case
if a phonological loop deficit were affecting vocabulary acquisition directly, leading Laws (1998) to suggest that
"The link between vocabulary and phonological memory in Down syndrome is not as well established as it is in normal development" (p. 1121).

Second, although rehearsal is certainly an important component of typical phonological loop functioning, there is growing evidence to suggest that a
problem in rehearsal may not be the prime cause of poor verbal short-term memory in Down syndrome. This is not to say that rehearsal is entirely unimpaired,
simply that problems in rehearsal may not be the main reason why individuals with Down syndrome show lower verbal short-term memory spans than other
individuals. In our recent work (Jarrold et al., 2000) we
compared the verbal short-term memory spans of a group of individuals with Down syndrome and a comparison group of individuals with learning disability but
without Down syndrome. In addition, individuals' speech rates were measured by asking them to repeat sequences made up of the words used in the memory
task as rapidly as possible. This provides a measure, albeit an indirect one, of the speed with which individuals should be rehearsing these words in the
memory task. The individuals with Down syndrome had lower memory spans than the comparison group, in line with the body of research outlined above, but both
groups had comparable speech rates. This implies that the problems in memory seen in these individuals with Down syndrome are not due to slower, more
inefficient rehearsal.

Of course this assumes that individuals with Down syndrome actually employ rehearsal to aid their verbal short-term memory. In fact there are reasons for
thinking that individuals with Down syndrome might be unlikely to use rehearsal at all. These follow from evidence that typically developing individuals do
not begin to rehearse until around the age of 7 years (see Gathercole, 1998). Although somewhat controversial,
according to this view one might not expect to see rehearsal emerge among individuals with Down syndrome until they reach the equivalent developmental level.
To assess this possibility we carried out a further study, in which the size of the word length effect was assessed in a 'probed recall' task. As
discussed above, poorer memory for words of a longer spoken duration - the word length effect - is potential evidence that rehearsal is occurring. However, a
word length effect might also arise because a sequence of long words takes longer to say when it comes to providing the response to a short-term memory test
(Cowan, et al.,
1992). To counter this latter issue, memory was assessed in a task where three words were presented and individuals were prompted to remember either the
first, the second, or the third word in the list. Under these conditions individuals with Down syndrome did not show a word length effect, suggesting that
they were not rehearsing. Their overall level of memory was poorer than a comparison group, but importantly these control individuals also showed no word
length effect in recall. This implies that a problem of rehearsal is not the main cause of the verbal short-term memory deficit in this instance, because
both groups showed no evidence of rehearsal, and yet the individuals with Down syndrome still had lower verbal short-term memory spans.

4. Key Questions for Future Work

The previous section highlighted two implications of the claim that Down syndrome is associated with problems of phonological loop function, but also
noted that both of these implications are not clearly supported by the existing evidence from studies of Down syndrome. A phonological loop deficit would
predict impaired vocabulary acquisition, but it is not clear that vocabulary learning is particularly problematic for individuals with Down syndrome.
Rehearsal is a key process in phonological loop functioning, but it does not appear that problems in rehearsal are the main reason why verbal short-term
memory is poor in Down syndrome. The two questions outlined here for future research follow from these two unresolved issues.

The first question is whether a phonological loop problem has consequences for learning in Down syndrome. Although the existing evidence suggests that
vocabulary is a relatively strong aspect of language functioning, there are few, if any, direct studies of vocabulary learning in Down syndrome. Chapman,
Kay-Raining Bird and Schwartz (1990) have examined
'fast mapping' abilities. This is the ability to learn or realise that an unfamiliar word must be paired with an unfamiliar object. Fast mapping is
tested, for example, by showing individuals two familiar and one novel object, and asking them to point to the 'koob' (or some other unfamiliar
term). Chapman et al. (1990) found that
individuals with Down syndrome had no difficulty with this task. However, this kind of task does not assess the precise aspects of vocabulary acquisition
that are thought to be supported by the phonological loop, as it does not really test individuals' ability to learn and remember the phonological form of
the novel word. Studies of this latter form of word learning have been conducted with typically developing children (e.g., Gathercole et al., 1990; Gathercole, et al., 1997), and could be
adapted for use with individuals with Down syndrome.

A second key question concerns the precise nature of the apparent phonological loop problem in Down syndrome. If it does not reflect a rehearsal deficit,
or problems of hearing and speech, then what is its cause? As outlined above, the phonological loop consists of two processes or components - rehearsal and
the phonological store. If the deficit is not primarily one of rehearsal, then one is left to conclude that it must reflect some form of impairment with the
phonological store. There are two problems with this suggestion. The first is that it is a rather 'post-hoc' account, and the second is that, as yet,
we have no clear and accepted method of measuring phonological store functioning or capacity. One possible way in which this might be done is to look at the
rate at which individuals forget verbal information over time. If individuals with Down syndrome do not rehearse, then the rate at which they forget
information should provide a reasonably direct index of the efficiency of their phonological store.

5. Conclusions

This paper began with a review of verbal short-term memory in Down syndrome. Taken together, the evidence suggests that individuals with Down syndrome do
have relatively poor verbal short-term memory abilities (although there are certainly individual exceptions to this rule), and that these appear not to be
caused by problems of hearing or speech. This suggests that there really is a fundamental short-term memory problem associated with the condition, which in
terms of Baddeley and Hitch's working memory model would be viewed as a phonological loop deficit. Two implications of this claim were outlined. First,
that this should predict problems of vocabulary acquisition in Down syndrome, and second, that the problem might well be one of inefficient or absent
rehearsal. However, it appears that neither of these suggestions is fully supported by the existing evidence, and future work is clearly needed to determine
the precise causes and consequences of a phonological loop deficit in Down syndrome.

Acknowledgements

The authors are supported by a grant from the Medical Research Council to Alan Baddeley and Susan Gathercole. We are grateful to Down Syndrome Education
International for their invite to present this paper at the 3rd International Conference on Language and Cognition in Down syndrome.

Das, J. P. & Mishra, R. K. (1995). Assessment of cognitive decline associated with aging: A comparison
of individuals with Down syndrome and other etiologies. Research in Development Disabilities, 16, 11-25.

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